1. Field of the Invention
The present invention relates generally to circuits for protecting power supplies from damage by current overloads, and more particularly to a circuit for protecting electrostatic precipitator power supplies utilizing ferroresonant transformers from damage caused by current overloads.
2. Background of the Invention
The use of electrostatic precipitators to remove particulate matter from an airstream is well known in the art. Typically, an ionizer is utilized to produce an electrostatic field to charge the contaminating particles. The charged particles may then be captured by a collecting cell comprised of charged and grounded plates to accumulate the charged particles or contaminants.
In order to achieve efficient operation of electrostatic precipitators of the type described, it is well known that relatively high voltage levels must be maintained on both the ionizer and collecting cells. As contaminants are accumulated by the precipitator collecting cells, it is not uncommon for intermittent arcing, which may progress to an essentially continuous or short circuit condition, to occur between the highly charged potential surfaces and ground within the collecting cell. Arcing of this type may result in unnecessarily high overload currents required of the precipitator power supply, which can damage electronic components and result in inefficient precipitator operation. For example, overload currents caused by collecting cell arcing may produce cell voltages less than that required to trap charged particles, resulting in large numbers of the charged particles being exhausted into the area to be treated. These problems have been found to be particularly acute in industrial operations where higher operating voltages and larger contaminate accumulations of varying conductivity result in more frequent arcing between precipitator components.
Several methods have been proposed to eliminate hazards caused by current overloads in electrostatic precipitator power supplies. For example, circuit breakers have been employed to disable the power supply when the primary current supplied to the precipitator exceeds a predetermined value. However, such circuit breakers can present a shock hazard since large currents may exist before the circuit breaker trips to disable the power supply. In addition, for precipitator arcing conditions less than a direct short, or in cases where the arcing condition is intermittent, the primary current level necessary to trip the circuit breaker may not be reached. Circuit breakers are also known to be subject to mechanical wear and, in general, are not adjustable to provide for variations in trip current.
Ferroresonant transformers have also been suggested to inhibit electrostatic precipitator power supply operation in the presence of overload current conditions. The output voltage of such transformers generally remains constant or decreases slightly with increasing current, until a critical point of ferroresonance is reached, whereupon the output voltage begins to decrease rapidly with small increases in output current. Once the critical point or "knee" has been reached, the output voltage of the transformer may continue to decrease even though the output current also decreases, producing the characteristic ferroresonant transformer foldback effect. Power supplies relying only on the voltage regulating characteristics of the ferroresonant transformer have not successfully dealt with protecting the power supply against current overloads, since such transformers fail to provide protection in the upper current ranges, particularly after the knee of the transformer voltage-current characteristic curve has been exceeded and transformer operation is continuing in the foldback region.